1. Field of the Invention
This invention generally relates to organic light emitting devices (OLEDs) and, more particularly, to an OLED with a spherical back mirror for improved light extraction.
2. Description of the Related Art
As noted in Wikipedia, an OLED (organic light-emitting diode) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound which emits light in response to an electric current. This layer of organic semiconductor is situated between two electrodes. Generally, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as mobile phones, handheld games consoles, and PDAs.
There are two main families of OLEDs: those based on small molecules and those employing polymers. Adding mobile ions to an OLED creates a light-emitting electrochemical cell, which has a slightly different mode of operation. OLED displays can use either passive-matrix (PMOLED) or active-matrix addressing schemes. Active-matrix OLEDs (AMOLED) require a thin-film transistor backplane to switch each individual pixel on or off, but allow for higher resolution and larger display sizes.
An OLED display works without a backlight. Thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions such as a dark room an OLED screen can achieve a higher contrast ratio than an LCD, whether the LCD uses cold cathode fluorescent lamps or LED backlight.
Multilayer OLEDs can be fabricated with two or more layers in order to improve device efficiency. As well as conductive properties, different materials may be chosen to aid charge injection at the electrodes by providing a more gradual electronic profile, or block a charge from reaching the opposite electrode and being wasted. Many modern OLEDs incorporate a simple bilayer structure, consisting of a conductive layer and an emissive layer. The emissive layer may be understood as comprising a light-emission or electron injection layer, and an electron transport layer. Likewise, the conductive layer may be understood as comprising a hole injection layer and a hole transport layer.
During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. A current of electrons flows through the device from cathode to anode, as electrons are injected into the lowest unoccupied molecular orbit (LUMO) of the organic layer at the cathode and withdrawn from the highest occupied molecular orbit (HOMO) at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes are generally more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO.
Indium tin oxide (ITO) is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the HOMO level of the organic layer. A typical conductive layer may consist of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) [PEDOT:PSS] as the HOMO level of this material generally lies between the workfunction of ITO and the HOMO of other commonly used polymers, reducing the energy barriers for hole injection. Metals such as barium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the LUMO of the organic layer. Such metals are reactive, so they require a capping layer of aluminum to avoid degradation.
FIG. 1 is a partial cross-sectional view of an OLED (prior art). Layer 1 represents a stack comprising an electron transport layer, electron injection layer, and a cathode electrode. Likewise, layer 2 represents a stack comprising a hole transport layer, hole injection layer, and the anode electrode. Electron emission occurs at the interface between layer 1 and layer 2. The figure depicts energy loss in OLED structure due to the internal total reflection of high index waveguide.
θc2 is the critical angle for the media interface of n3 and n2. θc1 is the critical angle for the media interface of n2 and n1. θc0 is the incident angle to metallic mirror when a ray reaches critical angle θc2. The index of refraction for organic layer 1, organic layer 2, and isolation layer are all close to n1, and n1>n2>n3.
            θ      ⁢                          ⁢      c      ⁢                          ⁢      1        =          arcsin      ⁡              (                              n            ⁢                                                  ⁢            2                                n            ⁢                                                  ⁢            1                          )                        θ      ⁢                          ⁢      c      ⁢                          ⁢      2        =          arcsin      ⁡              (                              n            ⁢                                                  ⁢            3                                n            ⁢                                                  ⁢            1                          )                        θ      ⁢                          ⁢      c      ⁢                          ⁢      0        =          arcsin      ⁡              (                              n            ⁢                                                  ⁢                          2              ·                              sin                ⁡                                  (                                      θ                    ⁢                                                                                  ⁢                    c                    ⁢                                                                                  ⁢                    2                                    )                                                                          n            ⁢                                                  ⁢            1                          )            
For example, if n1=1.75, n2=1.5, and n3=1.0 the critical angle of θc1≈59°, θc2≈42°, and θc0≈35°. A back reflecting mirror can collected particle light and enhanced extraction efficiency. However, only the back propagation of light at θ<θc0 can be extracted out. The back propagation of light at θ>θc1 is confined in isolation layer (n=n1). The back propagation light with θc0<θ<θc1 is confined in the media of index n1 and n2.
FIG. 2 is a plot of the external extraction efficiency of the structure of FIG. 1 plotted as a function of organic layer 1 (OL1) thickness using a finite-difference time-domain simulation (prior art). The light source is located at the interface of organic layer 1 and organic layer 2. The light source is an incoherent dipole source at a wavelength of 450 nanometers (nm). The extraction efficiency into substrate is ˜43% and that into air is close to 20% at 450 nm, with the thickness of layer 2 set to 250 nm.
It would be advantageous if the light extraction efficiency of an OLED could be improved.